Diaphragm cell chlorine production

ABSTRACT

Improved apparatus and process to electrolytically produce chlorine gas and an alkali metal hydroxide in a diaphragm cell. The improved process comprises circulating the catholyte and contacting a foraminous cathode with an oxidizing gas having a regulatably controlled moisture content.

United States Patent 3,923,628

Gritzner Dec. 2, 1975 [541 g g z ig CHLORINE FOREIGN PATENTS ORAPPLICATIONS 832,196 4/1960 United Kingdom 204/98 [75] Inventor: GerhardGritzner, Midland, Mich.

[731 Asslgnee: l Dow Chemical Company Primary Examiner-Howard S.Williams Midland, MlCl'l.

Assistant Examiner-W. 1. Solomon [22] Filed: Aug. 1, 1974 Attorney,Agent, or Firm-Robert W. Selby [211 App]. No.: 493,812

Related US. Application Data [62] Division of Ser. No. 361,743, May 18,1973. [57] ABSTRACT Improved apparatus and process to electrolytically[52] 204/237 l l produce chlorine gas and an alkali metal hydroxide in51 Int. cl. 625B 9/00 a dialhragm Cell' The improved f Comprises [58]FM! of l 261 265 culatmg the catholyte and contacting a foraminouscathode with an oxidizing gas having a regulatably 204/266 3; 136/136controlled moisture content.

[56] References Cited v UNITED STATES PATENTS 9 Claims, 2 DrawingFigures 2,681,887 6/1954 Butler, Jr. 204/265 X P0 wer' Source 1 542 W12a: E E 45 l; 52a Zea i 18a 5 8 Q jscz I I:

DIAPHRAGM CELL CHLORINE PRODUCTION CROSS-REFERENCE TO RELATEDAPPLICATION This is a division, of application Ser. No. 361,743 filedMay 18, 1973.

BACKGROUND OF THE INVENTION This invention pertains to the electrolyticproduction of chlorine in a diaphragm cell and more in particular to anelectrolytic cell containing an oxidizing gas depolarized cathode and amethod of producing chlorine and an alkali metal hydroxide in suchelectrolytic cell.

Gaseous chlorine has long been produced from sodium chloride in anelectrolytic cell havingan anode positioned within an anode chamber anda cathode in a cathode chamber spaced apart from the anode chamber by anion and liquid permeable diaphragm, such as one at least partiallyformed of asbestos. In such an electrolytic cell chlorine is released atthe anode and sodium hydroxide is formed in the cathode chamber.

Various methods to conserve electrical power in electrolytic cells havebeen developed using porous cathodes in combination with an oxidizinggas to depolarize the electrode; see for example, Juda, US. Pat. No.3,124,520. It is desired to provide an improved apparatus and process toreduce the electrical consumption of chlorine producing electrolyticdiaphragm cells.

SUMMARY OF THE INVENTION An improved electrolytic cell to producechlorine and an alkali metal hydroxide has been developed. Theelectrolytic cell comprises an anode compartment suited to contain ananolyte such as an aqueous solution or mixture of an alkali metalchloride, for example, sodium chloride. A cathode compartment adapted tocontain a catholyte containing the hydroxide of the alkali metal isspaced apart from the anode compartment by a diaphragm. The diaphragmseparating the anode and cathode compartments is suited to pass ions ofat least the alkali metal from the anode compartment to the cathodecompartment. The diaphragm is suitably positioned in the electrolyticcell to substantially entirely separate the anode compartment from thecathode compartment.

An anode is suitably positioned within the anode compartment and acathode is suitably positioned within the cathode compartment to bespaced apart from the diaphragm, that is substantially all of thecatholyte is contained within a space or opening at least partiallydefined by the diaphragm and at least partially by an outer surface ofthe cathode. The cathode is further adapted to have at least one wallportion in contact with the catholyte and at least one other wallportion substantially simultaneously in contact with an oxidizing gas.

A means to circulate the catholyte at least within the cathodecompartment'is in operative combination with the cathode compartment. Ameans to control the moisture content of the oxidizing gas in contactwith the cathode is in operative combination with the cathode.

A means to supply a direct current to the anode and the cathode issuitably electrically connected to these electrodes. The electrolyticcell further includes a means to remove the chlorine produced from theanode compartment and a means to remove the alkali 2 metal hydroxideformed from the cathode compartment.

The described electrolytic cell is advantageously used in an improvedprocess to produce chlorine and an alkali metal hydroxide. In theimproved process an alkali chloride brine is fed into the anodecompartment. At least a portion of the brine containing alkali metalions passes through the diaphragm into the cathode chamber. Sufficientelectrical energy is supplied to the anode and cathode to releasegaseous chlorine at the anode and to form the alkali metal hydroxide inthe cathode compartment. The gaseous chlorine and alkali metal hydroxideare suitably recovered by means known to those skilled in the art.

The electrical efficiency of the cell is improved by substantiallysimultaneously contacting different wall portions of the cathode withthe catholyte and with an oxidizing gas. The moisture content of theoxidizing gas is suitably controlled to minimize drying and depositionof materials such as sodium chloride, sodium hydroxide and the like onthe cathode surface. The catholyte is circulated within the cathodecompartment to maximize contact between the catholyte and the cathode tothereby further improve the electrical efficiency of the cell.

DESCRIPTION OF THE DRAWING The accompanying drawing further illustratesthe invention:

In FIG. 1 is depicted a cross sectional view of one embodiment of theinvention.

In FIG. 2 is a cross sectional view of another embodiment of theinvention.

Identical numbers, distinguished by a letter suffix, within the severalfigures represent parts having a similar function within the differentembodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An electrolytic cell 10 of FIG.1 includes an anode compartment 12 with an anode 14 positioned thereinjuxtaposed and spaced apart from a cathode compartment 16 with adepolarized cathode 18 positioned therein. The anode compartment 12 isspaced apart from the cathode compartment 16 by a diaphragm 20 capableof passing at least alkali metal ions from the anode compartment 12 tothe cathode compartment 16. The electrolytic cell 10 further includes asource of alkali metal chloride brine (not shown) and a means 22 tointroduce or feed the brine into the anode compartment 12. A gaseouschlorine removal means such as a pipe 24 is suitably connected to theanode compartment 12 to afford removal of gaseous chlorine withoutsubstantial loss of chlorine to the ambient atmosphere.

A means, such as a pump, ultrasonic vibrator or a turbine type impeller26, to circulate the catholyte at least within the cathode compartment16 is suitably positioned within the cathode compartment 16 to affordcirculation of the catholyte throughout the cathode compartment 16.During operation of the electrolytic cell 10 the catholyte containsincreasing concentrations of an alkali metal hydroxide, such as sodiumhydroxide, which for efficient operation should be removed from thecathode compartment 16 to reduce the hydroxide concentration. For thispurpose an alkali metal hydroxide removal means such as pipe 28 is incombination with the cathode compartment 16.

The cathode 18 is formed of a material adapted to transmit or pass anoxidizing gas from a gas compartment 32 to the outer surface of thecathode l8. Preferably, formation of oxidizing gas bubbles on the outersurface of the cathode 18 is minimized and more preferably the outersurface of the cathode is substantially free of oxidizing gas bubbles.An oxidizing gas moisture control means 34 is provided to regulatablycontrol the dew point of the oxidizing gas introduced into the gascompartment 32 to minimize and preferably substantially entirelyeliminate accumulation of liquid water within the gas compartment 32.The moisture control means 34 is further adapted to maintain theoxidizing gas moisture content at a concentration adequate to minimizeand preferably substantially entirely prevent removal of sufficientmoisture from the catholyte within the cathode compartment 16 to resultin deposition of solid materials such as sodium chloride or sodiumhydroxide in, for example, the pores of the cathode 18. Preferably themoisture control means 34 is adapted to regulate the moisture content ofthe oxidizing gas within the range of from about 50 to 100 percent ofsaturation.

The cathode 18, which is used in combination with the oxidizing gascontrol means 34, is preferably a foraminous body, such as a screen,expanded metal or a sheet with holes extending therethrough, having atleast the surface thereof composed of a substantially inert material.For example, the cathode can be a substrate with at least a surface ofRu, Rh, Pd, Ag, Os, Ir, Pt and Au. The inert material is then coatedwith a mixture of at least one of the aforementioned inert metals inparticulate form and for example, polytetrafluoroethylene,polyhexafluoropropylene and other polyhalogenated ethylene or propylenederivatives. Preferably the particulate inert metal is what is known inthe art as platinum black, silver black and carbon black. Particulateswhich are designated as black generally and preferably have a US.Standard Mesh size range of less than about 300. Preferably the cathode18 is a screen at least partially woven from or adherently coated withmetallic platinum, silver or gold with a mesh size of about 30 to about60.

A source of electrical energy 36 is electrically connected to an energytransmission or carrying means such as aluminum or copper conduit as busbar or cables 38 to transmit direct electrical current to the anode l4and the cathode 18.

In operation of the electrolytic cell an alkali metal chloridecontaining brine, such as sodium chloride, is supplied or fed throughthe brine feed means 22 into the anode chamber 12 wherein, throughelectrolytic processes known to those skilled in the art, gaseouschlorine is formed and removed through pipe 24 and thence to a chlorinecondensing and storage system (not shown). Sodium ions pass through theasbestos diaphragm 20 into the cathode compartment 16 wherein sodiumhydroxide is formed. An oxidizing gas, preferably oxygen, is fed intothe gas compartment 32 within the cathode 18 substantiallysimultaneously with the formation of the sodium hydroxide. The presenceof the oxidizing gas and the physical contact thereof with the innersurface of the cathode 18, while the outer surface of the cathode 18 issimultaneously in contact with the sodium hydroxide containingcatholyte, is believed to minimize and preferably prevent formation ofgaseous hydrogen in the cathode compartment 16 to thereby reduce theelectrical consumption and improve the electrical efficiency of thecell. Excess oxidizing gas is removed from the gas compartment 32through the oxidizing gas removal means or port 40.

To minimize what is believed to be formation of hydrogen at the cathode18 it is desirable that substantially all of the catholyte comes intocontact with the cathode. To promote such contact and reduce theoccurrence of stagnant portions of catholyte within the cathodecompartment 16 where little movement of the catholyte occurs, thecatholyte is preferably circulated at a rate sufficient forsubstantially all of the catholyte to contact the cathode 18 andinsufficient to result in physical injury to the asbestos diaphragm 20.

FIG. 2 is illustrative of an electrolytic cell 10a having therein ananode compartment or chamber 12a spaced apart from a cathode compartmentor chamber 1611 by an asbestos containing diaphragm 20a formed from, forexample, asbestos sheet or particulate. An anode 14a is suitablyattached in the anode chamber 12a. Likewise, a cathode 18a is suitablyattached in the cathode compartment 16a. The anode is constructed of amaterial such as carbon or what is known in the art as dimensionallystable anode such as titanium or tantalum coated or plated withmaterials including, for example, at least one metal or oxide of theplatinum group metals including Ru, Rh, Pd, Ag, Os, Ir, Pt and Au.

The cathode 18a is preferably a metallic silver plated foraminous coppersubstrate such as a copper screen or sheet with a thickness of about0.0l to about 0.02 inch and sufficient pores or holes with a diameter ofabout 0.015 to about 0.03 inch diameter extending therethrough toprovide a total hole or open area equivalent to about 20 to about 40percent of that portion of the copper sheet having the greatest surfacearea. The foraminous copper sheet is preferably coated or plated withsufficient metallic silver to provide a substantially continuous silverlayer with a thickness of up to about 0.002 inch. Plating of the coppersubstrate is carried out in a manner known to those skilled in theplating art. A screen woven from about 0.005 to about 0.02 inch diameterwire into a screen having a US. Standard Mesh size of about 20 to about50 is satisfactory when plated with silver as described above. Thesilver plated copper substrate is coated with a mixture of platinumblack, silver black or carbon black and, for example,polytetrafluoroethylene. or a fluorinated copolymer ofhexafluoropropylene or tetrafluoroethylene. The mixture preferablycontains from about 30 to about weight percent carbon black with a meshsize of less than about 300 admixed with up to about 10 weight percentcarbon fibers. The balance of the mixture is essentially the organicmaterial and impurities generally found in the carbon and the organicmaterial. The organic mixture coated, silver plated copper is preferablysubstantially impervious to passage of the catholyte. The term copperincludes commercially pure copper and alloys' thereof containing atleast 50 weight per cent copper.

The pump 26a together with appropriate conduits extending into thecathode chamber 16a are provided to afford effective circulation of thecatholyte during operation of the cell 10a. Generally the catholyte willbe pumped in a manner to enter at the upper portion of the cathodechamber 16a and be withdrawn at the lower portion of the chamber;however, pumping can be carried out to remove catholyte at the upperportion of the cathode chamber.

The cathode 18a is spaced apart from a side portion or wall 42 of thecell a to form an opening or gas compartment 32a between the diaphragm18a and the inner surface of the wall 42. An oxidizing gas with themoisture content suitably controlled by a moisture control means 34a ispumped into, preferably, the upper portion of the gas compartment 32aand flowed into intimate contact with the outer surface 43 of thecathode 18a and withdrawn through removal means a for disposal.

A brine supply means 22a and a chlorine removal means 24a are incombination with the anode chamber 12a. A sodium hydroxide containingcatholyte is generally removed through a conduit 28a. A source of directelectrical current 36a is electrically connected to electrical conduits38a which are in turn electrically attached to the anode 14a and thecathode 18a.

Operation of the electrolytic cell 10a is substantially the same as thatdescribed for the embodiment of FIG. 1 except that the catholyte ispreferably circulated within the cathode chamber by pumping through thepump 26a.

The following examples further illustrate the invention.

EXAMPLES 1-33 An electrolytic cell substantially as shown in FIG. 2 witha drawn asbestos diaphragm, a graphite anode and a 3 /2 inch by 3% inchcoated platinum screen depolarminum foil. The aluminum foil-screencomposite was compressed under a pressure of 2000 lbs. per square inchand simultaneously heated for 2 minutes at a temperature of 360C. Thecomposite was cooled and then placed in a 20 percent sodium hydroxidesolution to dissolve the aluminum foils. The composite cathode waswashed and dried before being positioned in the electrolytic cell withthe Teflon coated surface forming a wall portion of the gas compartment.

The cell was operated using an electrode area of 3.14 square inches foreach the anode and cathode. The spacing between the anode and cathodewas either 11/16 inch or 1 11/ 16 inches as shown in the Tables. Anaqueous brine containing about 300 grams per liter sodium chloride wascontinuously fed into the anode chamber and a sodium hydroxidecontaining cell effluent was removed from the cathode chamber. Althoughchloride gas was continuously removed from the anode chamber it wasunnecessary to remove any gaseous product from the cathode compartmentwhile the depolarizing cathode was functioning.

Operation of the cell was carried out in a manner known to those skilledin the art with the exception that either oxygen or air was pumpedthrough the gas compartment during operation. Tables I, II and 111describe the specific operating conditions and operating results. Fromthese results it is clear that the cell voltage was significantlyreduced when the cathode was depolarized with either air or oxygen.

TABLE I Example l 2 3 4 Anolyte NaCl (gm/liter) 301 299 300 317 Naclo(gm/liter) 0.017 1.14 2.62 0.323 acidity (pH) 2.98 4.04 temperature (C.)67 71 66 69 head 9.0 10.0 12.5 7.0 Catholyte NaOH (gm/liter) 101 120 127108 NaClO; (gm/liter) 0.017 0.425 0.901 0.187 temperature (C.) 70 72 6970 Chlorine Composition C1 (per cent) 99.21 98.41 97.97 99.28 CO (percent) 0.54 0.85 0.71 0.40 0 (per cent) 0.19 0.70 1.29 0.25 Distancebetween electrodes (inch) 1 11/16 1 ll/l6 l 1l/16 ll/16 Voltage (volts)2.46 2.92 3.63 1.98 Current (amp.) 1.50 2.2 3.0 1.50 Current density(amp/in 0.48 0.70 0.95 0.48 Chlorine efficiency (per cent) 98.55 96.95"96.1 98.70

Oxygen used as the depolarizing gas.

*Reduction in efficiency is attributed to the particular asbestosdiaphragm. "Vertical distance in inches between the higher anolyte uppersurface and the catholyte upper surface.

ized cathode was used in the examples. The depolarized cathode includedabout a mesh platinum metal screen which had been coated with Teflon byfirst spray coating a 3 /2 inch by 3 /2 inch piece of aluminum foil withsufficient duPont Teflon 30B to form a layer of 16 milligrams of Teflonper square inch of aluminum surface. The Teflon coated surface was thenoven dried for 1 minute at a temperature of 360C. A 2 /2 inch diameterportion of the Teflon coated surface and a similar area of a seconduncoated aluminum foil surface was coated with a mixture of metallicplatinum having a mesh less than 300, l milliliter of water and 0.053milliliter of Teflon 30B latex. After uniformly distributing the mixtureover the aluminum foil surfaces the coating was air dried and then curedby slowly heating to a temperature of 350C. The platinum screen was theninterposed between the coated surfaces of the alu- T (D-continuedCurrent Density Cell Temperature (C.) Current (amp/ Voltage ExampleAnolyte Catholyte (amp) sq.in.) (volts) Distance between electrodes was1 1 1/ 16 inches; oxygen was the depolarizing gas.

a means to remove the chlorine from said anode compartment; a means toremove the alkali metal hydroxide from said cathode compartment; and ameans to supply electrical energy to said anode and said cathode. 2. Theelectrolytic cell of claim 1 wherein said cathode is a foraminous bodyhaving at least the surface composed of a substantially inert materialselected from the group consisting of Ru, Rh, Pd, Ag, Os, Ir, Pt

'Distance between electrodes was 1 1 1 16 inches.

What is claimed is:

1. An electrolytic cell to produce chlorine and an alkali metalhydroxide comprising an anode compartment adapted to contain an anolytecontaining an alkali metal chloride;

a cathode compartment adapted to contain a catholyte containing analkali metal hydroxide, said cathode compartment being spaced apart fromsaid anode compartment by a diaphragm;

a diaphragm suited to pass at least alkali metal ions from said anodecompartment to said cathode compartment, said diaphragm adapted toseparate said anode compartment from said cathode compartment;

an anode positioned in said anode compartment;

a cathode positioned in said cathode compartment and spaced apart fromsaid diaphragm, said cathode having a wall portion adapted to be incontact with the catholyte and another wall portion substantiallysimultaneously adapted to be in contact with an oxidizing gas;

an oxidizing gas compartment at least partially defined by a surfaceportion of said cathode and adapted to contain an oxidizing gas;

a means to circulate the catholyte at least within said cathodecompartment in combination with said cathode compartment;

a means to control the moisture content of the oxidizing gas incombination with an oxidizing gas supply means and said cathode;

and Au with a coating of a mixture of particulate inert metal anclanorganic component selected from the group consisting ofpolytetrafluoroethylene and fluorinated copolymers ofhexafluoropropylene and tetrafluoropropylene.

3. The electrolytic cell of claim 2 wherein the foraminous body is ascreen.

4. The electrolytic cell of claim 1 wherein said cathode is a silvercoated, woven copper screen with a mesh size of about 20 to about 50with an overcoating of a mixture of platinum, silver or carbonparticulate and an organic component selected from the group consistingof polytetrafiuoroethylene, and fluorinated copolymers ofhexafluoropropylene and tetrafluoroethylene.

5. The electrolytic cell of claim 2 wherein said diaphragm containsasbestos.

6. The electrolytic cell of claim 1 wherein said diaphragm containsasbestos.

7. The electrolytic cell of claim 1 wherein the catholyte circulationmeans is a pump spaced apart from said cathode compartment andcommunicating with said cathode compartment.

8. The electrolytic cell of claim 1 wherein said moisture control meansis adapted to control the moisture within the range of from about 50 toabout per cent of saturation.

9. The electrolytic cell of claim 8 wherein the catholyte circulationmeans is a pump spaced apart from said cathode compartment andcommunicating with said

1. AN ELECTROLYTIC CELL TO PRODUCE CHLORINE AND AN ALKALI METALHYDROXIDE COMPRISING AN ANODE COMPARTMENT ADAPTED TO CONTAIN A CATHOLYTETAINING AN ALKALI METAL CHLORIDE; A CATHODE COMPARTMENT ADAPTED TOCONTAIN A CATHOLYTE CONTAINING AN ALKALI METAL HYDROXIDE, SAID CATHODECOMPARTMENT BEING SPACED APART FROM SAID ANODE COMPARTMENT BY ADIAPHRAM; A DIAPHRAGM SUITED TO PASS AT LEAST ALKALI METAL IONS FROMSAID ANODE COMPARTMENT TO SAID CATHODE COMPARTMENT, SAID DIAPHRAGMADAPTED TO SEPARATE SAID ANODE COMPARTMENT FROM SAID CATHODECOMPARTMENT; AN NODE POSITIONED IN SAID ANODE COMPARTMENT; A CATHODEPOSITIONED IN SAID CATHODE COMPARTMENT AND SPACED APART FROM SAIDDIAPHRAGM, SAID CATHODE HAVING A WALL PORTION ADAPTED TO BE IN CONTACTWITH THE CATHOLYTE AND ANOTHER WALL PORTION SUBSTANTIALLY SIMULTANEOUSLYADAPTED TO BE IN CONTACT WITH AN OXIDIZING GAS; AN OXIDIZING GASCOMPARTMENT AT LEAST PARTIALLY DEFINED BY A SURFACE PORTION OF SAIDCATHODE AND ADAPTED TO CONTAIN AN OXIDIZING GAS; A MEANS TO CIRCULATETHE CATHOLYTE AT LEAST WITHIN SAID CATHODE COMPARTMENT IN COMBINATIONWITH SAID CATHODE COMPARTMENT;
 2. THE ELECTROLYTIC CELL OF CLAIM 1WHEREIN SAID CATHODE IS A FORMINOUS BODY HAVING AT LEAST THE SURFACECOMPOSED OF A SUBSTANTIALLY INERT MATERIAL SELECYED FROM THE GROUPCONSISTING OF RU, RH, PD, AG, OS, IR, PT AND AU WITH A COATING OF AMIXTURE OF PARTICULATE UNERT METAL AND AN ORGANIC COMPONENT SELECTEDFROM THE GROUP CONSISTING OF POLYTETRAFLUORETHYLENE AND FLLUORINATEDCOPOLYMERS OF HEXAFLUOROPROPYLENE AND TETRAFLUOROPROPYLENE.
 3. Theelectrolytic cell of claim 2 wherein the foraminous body is a screen. 4.The electrolytic cell of claim 1 wherein said cathode is a silvercoated, woven copper screen with a mesh size of about 20 to about 50with an overcoating of a mixture of platinum, silver or carbonparticulate and an organic component selected from the group consistingof polytetrafluoroethylene, and fluorinated copolymers ofhexafluoropropylene and tetrafluoroethylene.
 5. The electrolytic cell ofclaim 2 wherein said diaphragm contains asbestos.
 6. The electrolyticcell of claim 1 wherein said diaphragm contains asbestos.
 7. Theelectrolytic cell of claim 1 wherein the catholyte circulation means isa pump spaced apart from said cathode compartment and communicating withsaid cathode compartment.
 8. The electrolytic cell of claim 1 whereinsaid moisture control means is adapted to control the moisture withinthe range of from about 50 to about 100 per cent of saturation.
 9. Theelectrolytic cell of claim 8 wherein the catholyte circulation means isa pump spaced apart from said cathode compartment and communicating withsaid cathode compartment.